1. Field of the Invention.
This invention relates to AC machines with rectified output voltage and, more particularly, to rectifying circuitry incorporating capacitor-assisted line commutation to improve the rectified output voltage and the machine current waveform characteristics while eliminating the need for damper windings within the AC machines.
2. Description of the Prior Art.
Converter-fed AC machines operating in conjunction with line-commutated rectifiers or inverters generally require tertiary or damper windings in their rotating member to reduce the commutation reactance of the AC machine. Typically, the cost of such a damper winding ranges from 5% to 25% of the rotor fabrication expenses, depending upon the machine's size and type of construction.
In conventional converter-fed AC machines, damper windings have two main functions: (a) reduction of the commutation overlap (i.e., the time interval during which current transfers from one phase to the next) by decreasing the stored magnetic energy associated with the machine's leakage flux, and (b) absorption of the power associated with certain undesirable current harmonics which would, otherwise, be present in the machine's main winding. In view of the added cost and complexity resulting from the use of damper windings, it would be preferable to perform the above functions by an alternate means.
For a typical generator producing an AC voltage and current, a rectifier assembly is required to change the output of the generator to a DC voltage and current which is then applied to a load. The generator may, by way of example, include a rotatively driven rotor assembly and three sets of stator phase windings spaced around the rotor. As the rotor is rotatively driven, a sinusoidally varying voltage and corresponding current is induced within each stator phase winding. Since the phase windings are spaced about the rotating assembly, these sinusoidally varying voltages are 120.degree. out of phase with one another for a three-phase arrangement. Similar symmetry conditions apply to an M-phase arrangement. The rectifier assembly with its six solid-state switches for a three-phase full-wave bridge connected arrangement functions to successively switch, in turn, each of the stator phase windings into electrical connection with the load.
The conducting paths, which embrace the AC sources, the loads, the interfaces, and the solid-state switches and their interconnections, invariably possess inductance. Thus for transfer of current, i.e. for commutation of current (or briefly: commutation) from one path to another to occur, in a finite time a potential of appropriate polarity must force the current in the path existing prior to commutation to zero and build the current in the incoming path to the appropriate level. Through the time interval, during which both the offgoing and incoming switches are conducting, the AC machine is exposed to a momentary line-to-line short circuit, which reduces its recitifed output capability. The time interval is referred to as the commutation overlap. In the type of converters herein contemplated, under at least some operating conditions, the commutations are driven by the defined voltage source potentials of the AC generator, and occur when the next switch in sequence is closed without the help of external circuitry and requiring no additional solid state devices. These commutations are often called line commutations, natural commutations or load commutations. The generic term "line commutation" will be used here to delineate these communications. Similar names apply to the converters, e.g. we will refer to line-commutated converters.
When a source potential is not of the proper polarity to drive a source commutation, another potential must be introduced to effect the commutation desired within the converter. These potentials are typically developed by the action of a capacitor acting as a current source. This capacitor, and the associated extra switching components together are called the commutating circuit. Commutating circuits are parts of the switches, and not parts of the switching matrices of the converter logic circuit. Commutations effected by commutation circuits are generally termed forced commutations, and converters which rely wholly or largely thereon are said to be forced-commutated or self-commutated. Forced-commutated converters do not display overlap, but line-commutated converters require no extra commutative circuitry, and therefore are generally less complex than their forced-commutated counterparts.
It is therefore desirable to develop the combination of a line-commutated converter and an AC machine in which the need for a damper winding in the machine rotor is eliminated, and which additionally eliminates the deleterious effects of commutation overlap thereby reducing both the initial and the operating costs of the system incorporating the AC machine.
Special considerations apply in the use of generating systems for use on aircraft. The service requirements are generally more stringent and weight reduction is an important factor. Current practice utilizes a polyphase (typically three phase) AC generator in conjunction with line-commutated rectifiers for converting to a DC voltage output.
Whenever the output voltage of an aircraft generator is rectified, there is a significant voltage drop through the generator/rectifier unit. A part of this voltage drop is caused by the generator commutation reactance which prevents instantaneous current transfer, i.e., commutation, between rectifier branches. The main component of the commutation reactance is the generator leakage inductance. During commutation overlap, the generator terminals are subjected to line-to-line short-circuit (typically six times per cycle) which, in turn, reduces the output voltage by as much as 30%. In accordance with the present invention, static capacitors are connected to the AC terminals of the generator (built with or without damper windings) provide an alternate current path and thereby allowing current transfer from one phase to the next in sequence to be achieved instantaneously, thus substantially eliminating the commutation overlap and regaining the 30% drop in the output voltage.
Certain operating specifications require that aircraft generators be capable of handling 150% or higher rated load for several seconds. For such overload conditions, the size of the generator is dictated by the allowable rectifier drop resulting from the commutation overlap, rather than by the machine's thermal rating. Since the present invention eliminates commutation overlap, the mismatch between the machine's shorttime overload thermal capability and its rectified output is alleviated. In addition, since the capacitors can eliminate the commutation overlap, there is no need to employ damper windings for the purpose of keeping the machine's commutation reactance small. As a consequence, line commutated AC converter systems in accordance with the present invention can produce 25%-30% higher output per unit of equipment weight than can be achieved with conventional equipment, but with generators that are built without the extra cost of damper windings.
A number of favorable solutions to the basic problem of using an AC machine with line-commutation have been developed heretofore. Two particular examples of the use of an AC machine with a line-commutated inverter are disclosed in U.S. Pat. Nos. 4,445,081, entitled "Leading Power Factor Induction Motor Drive", of Kalman et al, and U.S. Pat. No. 4,476,424, entitled "Variable Speed Induction Motor Drive System", of Kalman, which are hereby incorporated by reference. A further solution to the problem of providing variable speed induction motor drive systems capable of operating with large induction motors of standard configuration is disclosed in U.S. patent application Ser. No. 520,093, entitled "Capacitor-Assisted Line Commutation for Induction Motor Drive", filed Aug. 4, 1983, of Kalman and Huggett, which application is assigned to the assignee of the present application and is hereby incorporated by reference.
In addition, numerous examples of related control circuits for variable speed AC machines may be found in the prior art. A typical example is the disclosure of Steigerwald in U.S. Pat. No. 4,039,926, entitled "Current Fed Inverter with Commutation Independent of Load Inductance". That disclosure involves a three phase bridge inverter providing rectangular wave currents to an inductive load and to capacitors which are connected in wye across the load for wave shaping as well as filtering and power factor correction. A force commutating capacitor connected to the midpoint of the wye-connected capacitors is controlled by two auxiliary thyristors. The force commutating capacitor voltage is sensed and an incoming thyristor is not fired until the voltage rises to a level sufficient to commutate the next thyristor in sequence, thereby providing forced commutation which is independent of load inductance. This force commutating capacitor, dual thyristor forms the switching circuit which acts to force commutate the bridge inverter. Although portions of the circuit of FIG. 1 of the Steigerwald patent appear superficially similar to figures depicting the invention herein, the circuit is force commutated and the wye-connected capacitors are provided for an entirely different purpose from the commutator capacitors incorporated in the present invention and no suggestion of the arrangement of the present invention and the beneficial results from the practice thereof has been found in the Steigerwald patent.